This invention is directed to a method and apparatus for protecting the basic refractory shapes in selected critical wear areas in the working lining of a basic oxygen furnace from early failure due to spalling caused by thermal shock. Specifically, this invention is directed to protecting the basic refractory shapes in critical wear areas such as the trunnion areas of a working lining in a basic oxygen furnace from early failure due to spalling caused by thermal shock during "burn-in" and during the early part of a furnace campaign.
A basic oxygen furnace is comprised of a metallic shell and several layers of basic refractory shapes which serve to protect the shell from the internal environment of the furnace, i.e. the heat and atmosphere generated inside the furnace during the production of a heat of steel. The innermost layer of the basic refractory shapes is called the working lining because it is exposed to the molten metal, heat and gaseous environment formed in the furnace. The working lining is comprised of a plurality of courses of tar bonded, tar bonded-tempered, or tar impregnated basic refractory shapes made from a basic material, such as sea water magnesite, naturally occurring magnesite, dolomite, magnesia and the like. Each of these basic refractory shapes has an outer coating of tar or tar-like carbonaceous material of greater or lesser thickness. Initially the tar on the surface of the brick allows the expansion of the brick thus reducing mechanical stresses which might lead to mechanical spalling. Subsequently the carbon in the tar reduces slag penetration during the steelmaking operation. The working lining is invariably laid-up without the use of mortar.
Prior to using the basic oxygen furnace for refining of raw materials to produce steel, the working lining is "burned-in" to prepare the lining for actual operation in producing steel. The "burn-in" comprises heating the working lining to a high temperature of about 2000.degree. F. (1093.degree. C) at a rate of about 25.degree. F. per minute without a charge of scrap or hot metal in the furnace. The exact rate of heating depends somewhat upon the particular shop practice. The "burn-in" serves in effect to cure the refractories, and in particular the tar material within and between the refractories, to increase the resistance of the refractories to slag penetration and in the case of an untempered or unbonded brick to prevent the refractories from slumping or collapsing during subsequent use of the furnace for the refining of steel. In the process, the tar coating on the exposed faces of the refractories melts and volatilizes exposing the hot faces of the basic refractory shapes to high temperatures. The exposed faces of the basic refractory shapes, generally referred to as the hot faces of the basic refractory shapes, are heated rapidly while the area of the basic refractory shapes immediately behind the hot faces remains relatively cold. A sharp thermal gradient is thus formed between the hot faces and the remainder of the basic refractory shapes. The sharp thermal gradient causes thermal shock in the refractories, which thermal shock frequently causes a portion of the basic refractory shapes to spall. The depth of the spall can involve as little as 2 inches or as much as 5 inches of the outer portions of the basic refractory shapes adjacent to the working faces. Spalling causes a reduction in the total amount of refractory material available to protect the furnace structure in the spalled areas of the working lining. As a result, the full life of the basic refractory shapes in the working lining is not utilized. Even where spalling does not immediately take place incipient or even actual cracks may be induced in the refractories which cracks lead to premature spalling of the refractories.
A more detailed, though still simplified, explanation of the occurrence of spalling caused by "burn-in" of the furnace follows. During burn-in the exposed end or face of a working lining of unplated basic brick, generally referred to as the "hot face", is heated rapidly, while the opposite end of the brick, generally referred to as the "cold face", is heated very slowly. Consequently, there is a large temperature gradient, which is non-linear, between the hot and cold ends of the brick, and the portions of the brick closest to the hot face are relatively hot while the portions closest to the cold face are relatively cold. The non-linear temperature gradient causes differential expansion of the brick giving rise to differential stresses in the refractories. When such stresses exceed the strength of the brick cracking occurs at varying distances behind the hot face, and severe cracks cause portions of the hot end of the brick to break away. This breaking or cracking action is referred to as "spalling". Obviously, the performance of a severely cracked or spalled brick is substantially poorer than the performance of a similar undamaged brick. The brick damaged by cracking tends to wear faster for a variety of reasons. The most obvious reasons are the reduced thickness of the brick, slag penetration into the cracked areas, additional spalling due to temperature cycling, and the like. However, even though spalling is not evident a certain amount of thermal shock damage will occur during accepted "burn-in" practices. A typical brick working lining is about 36 inches thick and spalling of the brick may significantly reduce its thickness during "burn-in". Obviously a brick of reduced thickness will not wear as well as an undamaged brick of full thickness.
Laboratory studies have confirmed that considerable damage occurs to refractories during "burn-in" even when spalling is not visibly evident. Lining bricks that were heated in the laboratory at normal plant "burn-in" rates of 25.degree. F/minute suffered up to a 65% loss in strength and cracking up to 5 inches in depth from the hot face of the brick. Further testing showed that by reducing the heat-up rate to 5.degree. F/minute it is possible to retain twice as much strength in the bricks as with the 25.degree. F/minute rate.
Although laboratory studies thus demonstrated the value of more moderate heat-up rates, there are other considerations in "burning-in" a furnace. For example, lowering the heat-up rate to 5.degree. F/minute would often adversely affect less expensive refractory bricks which may be used in some portions of the furnace though not throughout a furnace lining. Moreover, with a slower heatup rate, more production time is lost during "burn-in". Four to six heats could be tapped in the extra time it would take to heat-up a lining at a rate of 5.degree. F/minute as opposed to a 25.degree. F/minute heat-up. Given these practical considerations it is evident that some alternative method of reducing the heat-up rate in localized high-wear areas, such as for example the trunnion areas, while maintaining an overall heat-up rate of about 25.degree. F/minute in a basic oxygen vessel is required.
The basic refractory shapes in the hearth or dished bottom of the furnace are customarily protected from cyclic changes in temperature during operation of the furnace and from errosion of the refractories by the hot metal by reason of the slag retained in the bottom of the furnace between heats. The refractory shapes or bricks in the hearth thus do not tend to wear as rapidly as other basic refractory shapes in the working lining. The basic refractory shapes at the slag line are subjected to severe conditions because of the acid character of the early slag. However, these basic refractory shapes are protected by the carbon contained in the refractories and by a coating of basic slag formed later in the steelmaking process. Other basic refractory shapes in the furnace are coated with the basic slag by "rocking" the basic oxygen furnace on its trunnions after tapping the molten steel, but while a substantial amount of basic slag remains in the basic oxygen furnace. However, the basic refractory shapes of the working lining in the trunnion areas of the furnace and also in the cone areas of the furnace on the drive and idler side cannot be effectively coated with the basic slag because the conventional basic oxygen furnace cannot be turned so that the protective slag flows into these areas. Hence, the basic refractory shapes in these areas, i.e. the trunnion areas in particular and also the referred to cone areas, are more susceptible to early failure than the other basic refractory shapes in the furnace. Any cracks or incipient cracks occurring early in a campaign or early in the furnace use in the critical wear areas can easily lead to later spalling and excessive erosion in these areas. In those areas of the furnace in which the refractories can be protected by varous means during the making of a heat of steel in the furnace and particularly where the exposed surfaces of the refractories can be coated with a heavy layer of slag and especially with a basic magnesia containing slag, on the other hand, the early damage caused by incipient cracks and the like can to a large extent be effectively counteracted by such later protection. It is, therefore, essential that the basic refractory shapes in the critical wear areas be protected from excessive early erosion or corrosion or other early damage, such as incipient cracking of the refractories. Efforts, such as spraying the hot faces of the basic refractory shapes with a basic refractory material either prior to or immediately after "burn-in", have not proven to be entirely successful in protecting these areas. Spraying the hot faces prior to "burn-in" has not helped because the tar coating of the refractories melts during "burn-in" and any basic refractory material which has been sprayed on the refractories immediately falls away from the hot faces. Spraying after "burn-in", on the other hand, does nothing to prevent spalling due to thermal shock and resulting incipient cracking caused during "burn-in" and also during the early part of a furnace campaign since it takes several heats usually before a layer of spray material can be made to adhere to the furnace wall. There has been, therefore, a need for a simple, effective, yet inexpensive method of protecting the hot faces of the basic refractory shapes in critical wear areas, e.g. the trunnion areas in particular, and also in the drive and idler cone areas of the working lining in a basic oxygen furnace from thermal shock and spalling during "burn-in" and the early part of a furnace campaign.
It is the object of this invention to provide a method and apparatus for protecting the basic refractory shapes in critical wear areas of a working lining of a basic oxygen furnace from thermal shock and spalling during "burn-in" and the early part of a furnace campaign, which are relatively simple and inexpensive and which will alleviate the problems noted above.
It is a further object of this invention to provide a method for protecting the basic refractory shapes of a working lining in the trunnion areas of a basic oxygen furnace during "burn-in" and the early part of a furnace campaign, which method includes inserting a plurality of anchors in selected joints between the basic refractory shapes in a working lining during installation with one end of the anchors extending beyond the hot faces of the basic refractory shapes, securely fastening a reticulated reinforcing means, such as a metallic grid to the anchors, in spaced relationship to the hot faces of the basic refractory shapes and prior to "burn-in" spraying a coating of a pulverulent basic refractory material over the hot faces of the basic refractory shapes to a depth whereby the anchors and the reticulated reinforcing means are completely covered by the sprayed basic refractory material.